Three young bison bulls donated by media tycoon Ted Turner from his New Mexico herd will be introduced into the Texas Bison Herd at the Caprock Canyon State Park next summer. It is hoped they will provide much needed genetic diversity.
The Texas Bison Herd originated in the late 1800s with five bison calves captured by famed cattleman Charles Goodnight. The herd was donated to the state in 1997 and moved to the park. More information about the herd is available from http://www.
But even with intensive management by the Texas Parks and Wildlife Department that includes annual vaccinations, supplemental feeding and veterinary care, the herd has produced a small number of calves over the last six years. Possibly more troubling, the average age of approximately 40-animal herd has increased by three years, said Dr. James Derr, associate professor of veterinary pathobiology with the Texas A&M University College of Veterinary Medicine and the Texas Agricultural Experiment Station.
Derr and Dr. Natalie Halbert, a post doctoral research associate in the College of Veterinary Medicine, were asked to help.
"If you have a healthy, stable population that's having a reasonable number of offspring and the old ones are dying ... the average age should not significantly increase over time," Derr said. "If the population is expanding and more babies are being born than old ones are dying, the average would actually decrease.
"We, as geneticists, and the Texas Parks and Wildlife Department as wildlife managers, had a concern that something wasn't going right. The managers were doing everything they could, and the herd was not increasing in size."
While sampling the DNA and doing pregnancy tests in the fall of 2001, Derr and Halbert found that 15 of the 18 adult females were pregnant. However, by spring, when the females were due to calve, only one calf survived. The rest were either not born or did not survive long after birth.
Disease and genetic problems such as chromosomal defects were ruled out.
But since Derr and Halbert already were sampling the DNA from the federal bison herds throughout the United States for another project, they knew how much genetic variation there should be in an "average" bison.
"When we compared genetic variation in the Caprock Canyon bison herd, we found out they had significantly less genetic variation than any of the federal herds and most any of the other state and private herds," he said.
This led them to conclude the herd was suffering from inbreeding depression. Since the herd was confined on the Goodnight Ranch and then at the state park, no new genes had been brought into the herd in 120 years.
With the assistance of Dr. William Grant, a professor of wildlife and fisheries science for the Experiment Station, Halbert developed and used computer models to simulate the future for the herd under best-case and worst-case scenarios.
She used genetic and demographic information such as the natality (birth) rate and death rate based on data from the last several years in the model and examined the data year by year.
"The idea is to have an estimate, not a definite, of what would happen. In the future is the population going to be driven to extinction? Or is it likely to recover? Given the current problems with natality and mortality and the lack of variation at present, it's most likely in the next 50 years the population is probably not going to survive," she said.
One solution is to bring in new genetic diversity from an outside source.
"But in this herd, it's not something you do lightly," Derr said.
First of all, this herd, since it clearly originated in the Texas Panhandle, is probably the last genetic example of what was called the Southern Plains bison, said Danny Swepston of Canyon, Texas Parks and Wildlife Department wildlife district leader for the Panhandle. When the Transcontinental Railroad was built across the United States in the 1800s, the bison were split into what was known as the Northern and the Southern herds, with the latter made up of animals from Texas, eastern New Mexico, eastern Colorado, Kansas, Oklahoma and southern Nebraska. No other animals but those from Texas have been brought in.
"What we didn't want to do is bring animals in from outside," he said.
Also, the historical link to Goodnight made researchers reluctant to tamper with the herd too much, he said. Even though Goodnight donated three bulls to Yellowstone National Park before his death, animals could not be brought back from there because that herd is infected with brucellosis, a contagious bacterial disease, he said.
"Our best option was to try and find a herd that that has a historical link to Texas bison, that did not have hybrids (of cattle and bison) or diseases that could be transmitted to this herd, and had a lot of genetic variation so that the animals we bring in can immediately bring in new genetics to help overcome inbreeding depression," Derr added.
They found that with Turner's herd. "We tested that herd a few years ago, and we didn't find any evidence of cattle mitochondrial DNA," a sure sign of cattle genetics in the herd.
So last year, Texas Parks and Wildlife representatives, along with Texas A&M researchers traveled to New Mexico and chose three 1½-year-old bison bulls.
"We picked the three handsomest, orneriest, teen-aged bulls that were in there," Derr said, laughing.
The bulls arrived in January 2003 and have been placed in quarantine as a precaution against disease, Swepston said.
Texas Parks and Wildlife Department staff will keep them separated from rest of the animals until the cows are bred next summer. One or two of the young bulls will be put in with three or four cows, and samples of the DNA of the offspring will be collected.
Swepston said it will be a long-term project. "It will be probably be a few years before we know the results."
This type of work is important, not only to bison but other wildlife as well, Derr said.
"The kind of genetic technology we're using with North American bison ...was first developed for humans, and secondly used on livestock species such as cattle," he said.
Our hope is that when these studies are completed, they will form a model of the kinds of studies using genetic technology that can be used to preserve, conserve, and reconstruct many other wildlife species populations."
Two scientific articles explaining the details of this study in the Journal of Mammalogy and in Ecological Modelling were published this month.